1. Technical Field
This invention relates generally to roll forming of gears and more particularly to roll form densification of powder metal gear preforms and roll form burnishing of wrought gears.
2. Related Prior Art
It is known to produce gears from wrought (i.e., cast and/or forged) metal blanks and from pressed and sintered powder metal preforms. For many applications, powder metal gears offer a low cost alternative to wrought gears. Powder metal gears can be formed to near net shape, avoiding costly machining required to produce wrought gears. However, a principle disadvantage associated with powder metal gears made according to conventional practice is that they do not possess the high strength characteristic of wrought gears. The lower strength of powder metal gears is often attributed to low density resulting from traditional manufacturing processes. Consequently, powder metal gears generally have not been regarded as a practical alternative to wrought gears in applications such as transmission gear components which call for high strength precision gears.
The conventional manner of forming powder metal gears involves first pressing and sintering a preform to near net shape and thereafter roll forming the preform to density the teeth. The densification is carried out by enmeshing the powder metal preform with a toothed roll forming gear with sufficient force to densify the preform teeth. U.S. Pat. No. 5,711,187 discloses the general arrangement wherein a spur gear powder metal preform is supported on a spindle for free rotation about a first axis. A spur gear roll forming tool is driven about a second, parallel axis and meshes with the preform to in turn rotate the preform. Greater force is exerted on the driven side of the preform teeth than on the opposite coast side, resulting in corresponding variation in the densification of the teeth. The same difficulties arise in roll forming helical gears.
Further variation can occur as a result of the nature in which the gear teeth of the tool and preform interact. The teeth of the forming tool initially engage the driven side of the preform teeth and advance radially inwardly, exerting a radially inward wiping or rolling force on the driven side to effect densification. Through further rotation, the teeth of the tool confront the coast side and advance radially outwardly, exerting a lesser and oppositely directed wiping or rolling force on the coast side, leading to the variation in density. Some of the problem can be alleviated by reverse roll forming in the opposite direction, however this operational step adds extra time, cost and complexity to the forming of gears and may not eliminate the problem entirely.
In roll forming involute teeth of powder metal preforms, a trial and error approach is typically used to determine the proper sizing and tooth geometry of the rolling die need to produce the desired tooth profile, particularly an involute profile, as there is no mathematical basis known using current technology. If it is desired to alter the densification profile of the preform teeth, it is necessary to change roll forming dies or alter the starting profile of the preform teeth. As disclosed in the aforementioned '187 patent, it is sometimes necessary to employ a number of different forming die configurations to generate a desired densified tooth profile.
A similar technique is often used to burnish the teeth of wrought gears with similar difficulties being encountered with respect to burnishing as are encountered with powder metal densification. The solution thus far has been to reverse roll form the wrought gears, but at the cost of adding an extra step of complexity to the gear forming operation.
An object of the present invention is to provide a gear roll forming apparatus and method which simplifies the roll forming operation of powder metal and wrought gears while minimizing or altogether eliminating some or all of the disadvantages associated with known traditional processes.